Remove bad daemon connections from the pool

This is necessary for long-running processes like hydra-queue-runner:
if a nix-daemon worker is killed, we need to stop reusing that
connection.

src/libstore/remote-store.cc

@@ -40,7 +40,11 @@ template PathSet readStorePaths(Source & from);
 
 
 RemoteStore::RemoteStore(size_t maxConnections)
-    : connections(make_ref<Pool<Connection>>(maxConnections, [this]() { return openConnection(); }))
+    : connections(make_ref<Pool<Connection>>(
+            maxConnections,
+            [this]() { return openConnection(); },
+            [](const ref<Connection> & r) { return r->to.good() && r->from.good(); }
+            ))
 {
 }